1. Field of the Invention
This invention relates to hydraulic systems and more particularly to a hydraulic steering system for a vehicle.
2. Description of the Prior Art
Hydraulically powered steering systems are frequently employed on vehicles, such as large earthmoving tractor-scraper vehicles and the like. One type of steering system frequently employed on such tractor-scraper vehicles utilizes two positive displacement pumps of different volumetric capacities and a control valve with a pair of inlets individually connected for receiving fluid from each pump separately. The control valve is adapted to use the small pump for maintaining a continuous minimum pressure on the steering jacks to provide quicker steering response and for making small steering corrections. The output of the large pump is freely unloaded to tank, except when being utilized for making large steering corrections which greatly enhances operating efficiency. Typical examples of this type of steering system are those disclosed in U.S. Pat. Nos. 2,614,644; 2,846,848, and 3,154,921.
The addition of an auxiliary pump to the above described two pump type steering system is quite difficult and encumbered with unique problems not found in other types of steering systems. Firstly, since there is no common source of fluid pressure, the output of the auxiliary pump cannot be merely combined with such common source. It is also not advantageous to just simply combine the output of the auxiliary pump with that of one or the other of the large or small pumps.
For instance, if the output of the auxiliary pump were combined solely with the small pump, the auxiliary pump would be made to work against a continuous back pressure because the output of the small pump is utilized to maintain the continuous minimum pressure on the jacks, as mentioned previously. This, of course, would adversely effect operating efficiency. Conversely, if the auxiliary pump were combined solely with the large pump, all of the output of the auxiliary pump would be unloaded to tank with the output of the large pump, except when large steering corrections are being made. Thus, in the event of a failure of the small pump, no fluid would be available for making small steering corrections and would adversely effect the normal steering characteristics of the steering system. As is obvious, this could further contribute to the hazards of an emergency situation and negate any safety benefit which the auxiliary pump is intended to provide.